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SLVS456C − OCTOBER 2003 − REVISED OCTOBER 2004
10µF. A high quality ceramic type X5R or X7R is
recommended. The voltage rating should be greater than
the maximum input voltage. Additionally some bulk
capacitance may be needed, especially if the TPS54350
circuit is not located within about 2 inches from the input
voltage source. The value for this capacitor is not critical
but it also should be rated to handle the maximum input
voltage including ripple voltage and should filter the output
so that input ripple voltage is acceptable.
For this design example use K
inductor value is calculated to be 8.98 µH. The next highest
standard value is 10 µH, which is used in this design.
= 0.2 and the minimum
IND
For the output filter inductor it is important that the RMS
current and saturation current ratings not be exceeded.
The RMS inductor current can be found from equation 12:
2
ǒ
OUTǓ
0.8Ǔ
V
ǒVIN(MAX)
V
* V
IN(MAX)
OUT
1
12
I2
ǸOUT(MAX)
I
+
)
L(RMS)
L
F
OUT
SW
(13)
This input ripple voltage can be approximated by equation
9:
and the peak inductor current can be determined with
equation 13:
I
0.25
OUT(MAX)
) ǒ
MAXǓ
DV
+
I
ESR
IN
OUT(MAX)
C
ƒsw
BULK
(10)
ǒ
OUTǓ
F
V
V
* V
IN(MAX)
OUT
1.6 V
I
+ I
)
(14)
Where I
switching frequency, C
is the maximum load current, ƒ
is the
L(PK)
OUT(MAX)
OUT(MAX)
SW
L
IN(MAX)
OUT
SW
is the bulk capacitor value and
BULK
ESR
capacitor.
is the maximum series resistance of the bulk
For this design, the RMS inductor current is 3.01 A and the
peak inductor current is 3.34 A. The chosen inductor is a
Vishay IHLP5050CE-01 10 µH. It has a saturation current
rating of 14 A and a RMS current rating of 7 A, easily
meeting these requirements. A lesser rated inductor could
be used, however this device was chosen because of its
low profile component height. In general, inductor values
for use with the TPS54350 are in the range of 6.8 µH to
47µH.
MAX
The maximum RMS ripple current also needs to be
checked. For worst case conditions, this can be
approximated by equation 10:
I
OUT(MAX)
I
+
CIN
2
(11)
In this case the input ripple voltage would be 140 mV and
the RMS ripple current would be 1.5 A. The maximum
voltage across the input capacitors would be VIN max plus
delta VIN/2. The chosen bulk and bypass capacitors are
each rated for 25 V and the combined ripple current
capacity is greater than 3 A, both providing ample margin.
It is very important that the maximum ratings for voltage
and current are not exceeded under any circumstance.
Capacitor Selection
The important design factors for the output capacitor are
dc voltage rating, ripple current rating, and equivalent
series resistance (ESR). The dc voltage and ripple current
ratings cannot be exceeded. The ESR is important
because along with the inductor current it determines the
amount of output ripple voltage. The actual value of the
output capacitor is not critical, but some practical limits do
exist. Consider the relationship between the desired
closed loop crossover frequency of the design and LC
corner frequency of the output filter. In general, it is
desirable to keep the closed loop crossover frequency at
less than 1/5 of the switching frequency. With high
switching frequencies such as the 500-kHz frequency of
this design, internal circuit limitations of the TPS54350
limit the practical maximum crossover frequency to about
50 kHz. Additionally, to allow for adequate phase gain in
the compensation network, the LC corner frequency
should be about one decade or so below the closed loop
crossover frequency. This limits the minimum capacitor
value for the output filter to:
OUTPUT FILTER COMPONENTS
Two components need to be selected for the output filter,
L1 and C2. Since the TPS54350 is an externally
compensated device, a wide range of filter component
types and values can be supported.
Inductor Selection
To calculate the minimum value of the output inductor, use
equation 11:
ǒ
OUTǓ
V
V
* V
OUT(MAX)
IN(MAX)
1
K
L
+
2
C
+
(2pƒ )
MIN
V
K
I
F
OUT
L
IN(max)
IND
OUT
SW
(12)
CO
OUT
(15)
K
is a coefficient that represents the amount of inductor
Where K is the frequency multiplier for the spread between
f and f . K should be between 5 and 15, typically 10 for
LC
one decade difference.For a desired crossover of 50 kHz
and a 10-µH inductor, the minimum value for the output
capacitor is 100 µF. The selected output capacitor must be
IND
ripple current relative to the maximum output current. For
designs using low ESR output capacitors such as
ceramics, use K
CO
= 0.3. When using higher ESR output
IND
capacitors, K
= 0.2 yields better results.
IND
16
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